1. Field of the Invention
The invention relates to methods of use of orientation control compositions to impart controlled orientation of microdomains in films of block copolymers subsequently disposed on the orientation control layer.
2. Description of Background
Block copolymers are well known self-assembly systems, which form periodic microphase-separated domains (also referred to herein as both “microdomains” and “domains”) to minimize total free energy. Thin films of block copolymers provide spatial chemical contrast at the nanometer-scale (FIG. 1A) and, therefore, they have been used as an alternative low-cost nanopatterning material for generating periodic nanoscale structures. For example, lamellar forming block copolymers can align their domains either parallel (FIG. 1B) or perpendicularly (FIG. 1C) to the surface of a substrate surface. The perpendicularly oriented lamellae provide nanoscale line patterns, while there is no surface pattern created by parallel oriented lamellae. Where lamellae form parallel to the plane of the substrate, one lamellar phase forms a first layer at the surface of the substrate (in the x-y plane of the substrate), and another lamellar phase forms an overlying parallel layer on the first layer, so that no lateral patterns of microdomains and no lateral chemical contrast form when viewing the film along the perpendicular (z) axis. When lamellae form perpendicular to the surface, the perpendicularly oriented lamellae provide nanoscale line patterns. Therefore, to form a useful pattern, control of the orientation of the self-assembled microdomains in the block copolymer is necessary. Without external orientation control, thin films of block copolymers tend to self-organize into randomly oriented nanostructures (FIG. 1D) or undesired morphologies, which are of no use for nanopatterning because of the random nature of the features.
Orientation of block copolymer microdomains can been obtained by guiding the self-assembly process with an external orientation biasing method such as by use of a mechanical flow field, electric field, temperature gradient, or by the influence of surface interaction by use of a surface modification layer, with the layer of block copolymer. Of these, use of a surface modification layer for orientation control is relatively straightforward to integrate into a spin-casting or other film-forming manufacturing process, and is therefore desirable. Random copolymer brushes, thermally cross-linked random copolymers, and self-assembled monolayers have each been used as the basis of an orientation control layer to induce preferential orientation in block copolymer thin films.
While surface modification methods can be readily integrated into manufacturing processes, each has limits to its utility. Polymer brushes are difficult to tune to the desired thickness and surface energy, and require reactive precursors. Additional rinse steps are often required to remove non-bound material. The composition of random copolymer brush layers must be tuned precisely to afford a neutral surface. This can be achieved by randomly copolymerizing two monomers, for example the same monomers used in a block copolymer of interest, in a precise ratio. However, many otherwise useful block copolymers (i.e., those that can form microdomains) exist for which it is unfeasible to synthesize random copolymers of repeating units of each block, for example because of different required polymerization mechanisms. End-group functionalization of polymers, or copolymerization with a third functionalized monomer, have been used to provide grafting sites. However, grafting efficiency is typically poor, requires impractically long annealing times, and is compatible with only a limited range of substrates [See e.g., P. Mansky, Y. Liu, E. Huang, T. P. Russell, C. Hawker, “Controlling polymer surface interaction with random copolymer brushes”, Science, 275, 1458, (1997).] Thermally crosslinkable underlayers based on, for example, vinyl benzocyclobutene has been found to resolve some of these issues, but often require extended thermal cure steps. [See e.g., Du Yeol Ryu, Kyusoon Shin, Eric Drockenmuller Craig J. Hawker, and Thomas P. Russell “A generalized approach to modification of solid surfaces” Science, 308, 236, (2005)]. Photopatternable underlayers based on random copolymers of the monomers of the block copolymer with an appropriate functional monomer, for example, monomers having azide, glycidyl or acryloyl groups, have been used but provide relatively low cross-linking efficiency (as measured by lengthy exposure times and/or lengthy bake/anneal steps) and can further require an extra rinse step to remove non-crosslinked materials. [See e.g., Joona Bang, Joonwon Bae, Peter Lowenhielm, Christian Spiessberger, Susan A. Given-Beck, Thomas P. Russell, and Craig J. Hawker, “Facile routes to patterned surface neutralization layers for block copolymer lithography”, Advanced Materials, vol. 19, p. 4552 (2007); Eungnak Han, Insik In, Sang-Min Park, Young-Hye La, Yao Wang, Paul F. Nealey, and Padma Gopalan, “Photopatternable imaging layers for controlling block copolymer microdomain orientation”, Advanced Materials, vol. 19, pp. 4448 (2007) (epoxy groups in underlayers)].